Eric Leonard, the acting superintendent at Andersonville National Historic Site, likes to offer some numbers:

• During the worst times at Andersonville, 100 prisoners died of malnutrition per day. 12,000 died in 14 months.
• At the Gulag Solotevsky, 6,000 died in 2 years.
• During the worst times at Birkenau, 2,785 were killed per day. 1.1 million dead in 4.5 years.

All prison populations were subject to overcrowding, malnutrition, and disease in similar doses.

The main distinction is that concentration camp/gulag prisoners were usually forced to labor for ~12 hours a day, whereas Civil War prisoners generally did not face that burden.

• Day in the Life at Andersonville
• Day in the Life at Auschwitz